Yellow artifacts when baking normals
Baking normals can be tricky. If the normal of the lowpoly object is roughly the same as the highpoly object, the bake will be that bluish-cyan-pink that we like. However, if the normals points in opposite direction, the bake will be the inversion of those colors, as the yellow-brown-red.

Then why does the normals point in opposite direction? That could happen if the highpoly object intersects itself. The baking process have difficulty to find the correct face, with the correct normal.

Here are some images with a good and bad case:





In the second case the highpoly mesh intersects with itself. The lowpoly object can't be projected on the highpoly with a good result.